000
FXAK68 PAFC 181350
AFDAFC
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 AM AKST Tue Dec 18 2018
.ANALYSIS AND UPPER LEVELS...
The upper level low over Southwest Alaska yesterday is finally
beginning to break down and open up into a trough which is digging
southeastward near Kodiak Island this morning. To the west of the
trough, strong northerly winds continue to move Arctic air
southward across the eastern Bering. The telltale signature of
that is the streaks of showers racing southward over the Alaska
Peninsula and out to the northern Pacific. Pockets of low stratus
persist along the foothills of the Alaska and Kuskokwim Ranges,
with more stratus moving south out of the Yukon Delta into Bethel.
Further east across Southcentral, a plume of upper level moisture
is moving right up Cook Inlet and across the Susitna Valley. It
appears this feature is confined to the upper levels. Radar is
showing all of the precipitation occurring around the area moving
southeast to northwest, indicating a different steering flow in
the midlevels. Meanwhile the near surface air is out of the north,
as indicated by the echoes near the Kenai Radar. Across the
southern Kenai, heavier snow shower activity in a separate band is
ongoing as well, producing steadier snow for much of the area
south of Clam Gulch. Broad lift throughout the atmosphere to the
east and northeast of the upper level trough is responsible for
the precipitation activity ongoing across Southcentral.
&&
.MODEL DISCUSSION...
The models remain in decent agreement over the next 2 days across
the Gulf, with lesser agreement by Wednesday as to where a
developing low over the Bering ends up. Nevertheless the fronts
and significant features with the Bering and Gulf lows are roughly
the same in terms of placement among all the models. As has been
the case for a week now, the main challenges will be timing
precipitation across Southcentral, and where any fog or low
stratus will form across Southwest Alaska. Forecast confidence is
moderate.
&&
.AVIATION...
PANC...Snow associated with an area of moisture will cause
deteriorating conditions this morning through lunchtime as the
precipitation moves from east to west over the area. The snow will
settle just south of town for the afternoon and early evening
hours, before potentially moving back to the north into Anchorage
for the evening. The heavier snow shifts south and east out of
Anchorage for good after midnight. While snowing, conditions will
be MVFR to IFR at times, but prolonged heavy snow is not expected.
There is considerable uncertainty as to how far south the snow
moves this afternoon, and as such there is some potential that
light snow continues throughout the day with only brief, periodic
breaks between snow showers. Winds will remain light throughout
the TAF period.
&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level low will slide into the western Gulf today. This
will bend the upper level flow from southerly to southeasterly. A
weak front will continue to develop in the northern Gulf, and this
southeasterly flow will push mid-level moisture into Southcentral.
This will bring chance or likely Pops to the much of the area,
with the exception of the Copper River basin which will remain
dry. At the surface, the offshore gradient will strengthen as
surface high pressure builds behind the low over southwest Alaska.
As the upper low moves north tonight into early Wednesday, the
front will also be pushed northward and bring chances of snow to
all of Southcentral. On Wednesday afternoon the upper low will
begin to weaken and move eastward. This will slowly dry things out
over the area.
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Tuesday
through Friday)...
Cold temperatures continue to be the weather story for southwest
Alaska. Lows will persist in the -20F range for inland locations
the next couple of days, with tonight expected to be the coldest.
Some locations may get down to around -25F, though around -20F
should do it for most areas. This is due to a pocket of cold air
that will make a push south over the area later today.
Temperatures will warm slightly beginning tomorrow, but the region
generally stays in a cold pattern through the week, with
temperatures remaining below zero.
The only other item of note is a band of precipitation that will
work its way over the Western Alaska Range tonight into tomorrow.
This area of precip will not work very far west and will just
bring some areas of light snow to the eastern Lower Kuskokwim
Valley.
&&
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3/Tuesday through Friday)...
The persistent northerly flow in the Bering Sea will finally give
way to a change in the weather pattern this week. The Eastern
Aleutians and eastern Bering will still see the northerly pattern
today, with gale force winds associated with it. Also of note,
wind direction and forcing is such that storm force gusts can be
expected in bays and passes on the Pacific side of the eastern
Aleutians. A shift in the weather is coming tomorrow though.
The Western Aleutians today will see a long occluded boundary
work its way east into the island chain. This low will bring
precip and gale force winds with it, but will also end the
northerly flow as it slides east. Tomorrow this low will push into
the Central and Eastern Aleutians, ending northerly flow there as
well and warming temperatures a bit. So, finally a change in the
weather pattern for this part of our forecast area.
&&
.LONG TERM FORECAST (Days 3 through 7)...
Thursday starts off the long term forecast with an upper level
trough over Prince William Sound and the northern Gulf and a
broad upper level low and cold air mass stretched across mainland
Alaska. Precipitation along the coastal areas will taper off
through the day and shift offshore overnight as the upper level
support weakens and leaves a remnant surface low drifting in the
Gulf. A low south of the eastern Aleutian Chain Thursday will make
its way east to be located south of the Gulf on Friday. With the
more southern track of this low, impacts will be primarily limited
to the offshore waters. As the low continues into the
southeastern Gulf through the weekend, offshore flow and gap winds
will increase across Southcentral Alaska with skies clearing and
a rather cold air mass settling in over the area. This cold
pattern with strong northerly winds through channeled terrain
looks likely to persist through the first half of next week as a
more southerly storm track keeps the strongest storms well south
of the area and directed into the Pacific Northwest.
Looking out west, another front will approach the western
Aleutians and Bering on Thursday. Additional lows spinning up in
response will keep the weather pattern active over the Aleutians
and Bering for Friday and Saturday as well. A smaller frontal
wave in the eastern Bering will bring the potential for some snow
and blowing snow to the western capes and Kuskokwim Delta coast
Thursday through Saturday while the remainder of mainland
Southwest Alaska remains generally dry. Heading into Sunday, a low
south of the Aleutian Chain will be the main weather-maker to
keep an eye on for placement and intensity as its front moves east
towards the AKPEN through Monday.
&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning: 130.
Gale Warning: 131 132 138 150 155 165 170 175 176 177 178 185 352
411 412.
Heavy Freezing Spray Warning: 160 180 181 185.
&&
$$
SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...JR/ALH